The use of horizontal wells in oil reservoirs is currently of high interest within the oil industry. Horizontal wells allow more reservoir surface area to be contacted and thereby reduce inflow pressure gradients for reasonable oil production rates. Alternatively, for typical pressure gradients within the wellbore region, the productivity of a horizontal well is greater than that in a vertical well.
Possible benefits of horizontal wells are currently being exploited in the Canadian tar sands. Reservoirs in Canada that may be categorized as immobile under reservoir conditions include the Cold Lake and Athabasca deposits.
Current practices for producing immobile tar sands include mining and steam stimulation by formation fracturing. However, mining is not practical below very shallow depths. Furthermore, steam stimulation by formation fracturing is not feasible in those reservoirs underlain by water aquifers and/or developed by closely spaced vertical wells.
Steam stimulation below fracture pressure in a vertical well is not practical due to very low injectivity of the formation to steam and to the very small area of reservoir contact. Increased area of contact can be achieved by the use of long horizontal wells (1,000 to 3,000 feet as compared to 30 to 100 feet for a vertical well). This increased contact allows more of the reservoir's area to be heated by steam injection. This results in more oil production due to an increased volume of the heated zone. Unfortunately, for the immobile tar sands, even when heated, injectivity may remain very low. Injection of a large steam slug into a horizontal well underlain by a water aquifer may result in a fracture into the aquifer.
Horizontal wells are typically used to condition a formation in one of two ways. One way is to circulate steam into the wellbore in order to heat up the wellbore and reservoir therearound. Two types of countercurrent steam circulation methods are generally used. In one method, steam is injected into a casing annulus and fluids are circulated back and produced to through the tubing the surface. In another method, steam is injected inside the production tubing and fluids are circulated back through the casing annulus. Although countercurrent steam circulation improves some of the heat transfer between the casing and reservoir, it lacks the capability of ensuring uniform distribution of heat along the entire length of a completed horizontal wellbore.
Another way of conditioning a formation is by steam injection. Steam injection is either through the casing side or through the tubing. However, unlike steam circulation, no fluids are produced back. Also, all of the injection steam enters the reservoir. Regardless of the steam injection method, steam and heat may not be distributed uniformly into the reservoir. The resulting uneven heating of the formation inhibits utilization of the entire horizontal section for production purposes.
Therefore, what is needed is a method for steam circulation or steam injection into a wellbore which ensures uniform distribution of steam and heat throughout the entire length of a wellbore into a reservoir or formation.